1. Field of the Invention
The present invention relates to data modems for transmitting and receiving data over a telephone line and more particularly to modems having a microprocessor to perform the required modulation, demodulation, filtering and control digitally.
2. Description of the Prior Art
It is well known to transmit digital data over voice grade telephone lines, satellite channels, microwave channels, and other band limited transmission paths by the use of data modems which encode binary data into a form capable of transmission over such communication channels, and for decoding the received signals from the channels. To obtain high data rates over band limited channels, it has been common to utilize efficient modulation systems such as differential phase shift keying (DPSK), a typical technique which may provide data rates up to 4800 bits per second. DPSK modulation may utilize an 1800 Hz carrier with 8 phases of the carrier coded to represent 3 data bits at a time. In the past, the encoding and modulation has been performed with large and relatively expensive analog circuits to provide the necessary processing, equalizing, and filtering to the desired accuracy. In recent years, modems have been developed in which the modulation and demodulation is accomplished by use of digital computations executed by microprocessors.
The digital approach permits the major elements of a modem to be implemented by large scale integration which greatly reduces the initial cost. The accuracy of the system is greatly improved by elimination of the lumped circuits and discrete components used in analog systems whose values can drift and which require close manufacturing tolerances. Typical microprocessor implemented data modems are disclosed in U.S. Pat. No. 4,263,670 to Sherman and U.S. Pat. No. 4,085,449 to Walsh, et al. However, an inherent problem in such prior art digital modems is the required speed of operation and expense of the processor's digital circuits. Where the various elements of the modem must be synthesized digitally; for example, transversal filters, scramblers and Grey code converters, trigonometric tables, equalizers and the like, and will require volatile and non-volatile storage of the data, the data storage devices must be addressed and utilized by the processor at very high speeds. For example, if a clock of 4.8 MHz is used, only 208 ns are available between clock pulses in which to perform all of the necessary digital operations required during that period. Therefore, in order to provide a digital modem capable of all the functions which can be obtained in an analog type modem, it is necessary to provide improved digital processing circuits that minimize the time and parts required for various operations. It is to this problem that the present invention is addressed.